Effects of tidal straining on the semidiurnal cycle of dissipation in the Rhine region of freshwater influence: Comparison of model and measurements

Abstract

The Rhine region of freshwater influence in the North Sea is a region of the coastal ocean that exhibits strong cross‐shore density gradients interacting with the progressive Kelvin tidal wave which generates mixing. When mixing is low enough to allow stratification to develop, the stratification modifies the tidal currents and generates a strong tidal shear that will strain the density gradients generating a semidiurnal cycle of stratification. Observations of turbulent kinetic energy dissipation ( ) during calm conditions show that the bottom half of the water column has a predominantly M 4 cycle. Higher values of occurring during flood flow are associated with the residual flow in that direction. Following the strong peak in dissipation near the bed, enhanced dissipation is apparent extending up the water column. In the upper part of the water column, shows a predominantly M 2 variation with the maximum occurring at a time when vertical stability is negligible and the effect of tidal straining is to create instability in the water column as the cross‐shore shear forces higher salinity water over fresher water. The inference is that potential energy released by straining is responsible for convective motions and, hence, the enhanced turbulent activity observed. The cycle of stratification and dissipation has been simulated using a one‐dimensional Mellor‐Yamada levels 2 and 2.2 turbulence closure model. The model correctly predicts the amplitude and phase of the stratification. While the model is in qualitative agreement in the description of the dissipation and its variability, the absolute values of appear to be too high.